A: This is pretty easy to find on the National Weather Service's web page. Just go to www.weather.gov/sacramento and click on the "Climate" tab just above the map. On the next page, click on "Preliminary Monthly Climate Data" under the "product" header. Then, select the desired location and timeframe.

Another neat way to find climate data for any date for any location can be found on the www.climate.gov web page. Just look for the green "past weather" interface.

A: Try this . . . http://bit.ly/nRsdCd

A: The Delta Breeze typically affects the Lower Sacramento and Northern San Joaquin Valleys. The strength of the Delta Breeze can vary considerably depending on how it sets up. Typically, the northern extent is somewhere in the Marysville-Yuba area. The southern extent of the Delta Breeze influence is usually between Stockton and Modesto.

A: Forecasting anywhere is tricky Steve. In fact, if you ask any meteorologist where is weather prediction most difficult, chances are they will tell you it's wherever they are forecasting.

One significant factor in making a forecast tricky is terrain. Mountains make for much more varied weather conditions than the flat lands. Also, the numerical models used to forecast the weather can, at times, struggle to depict the complexity that mountains introduce. Mountains block winds, trap cold air, channel moisture, enhance rainfall on windward areas and diminish rainfall on leeward slopes.

Another challenge on the West Coast is the lack of upstream land stations. This matters less and less as remote sensing (e.g. satellites) becomes more advanced. But, meteorologists look at upstream weather for clues as to what is coming. That is harder to do when your upstream is a giant ocean.

That said, predictions are very good here on the West Coast. The accuracy our forecasts have today would have been unimaginable just 20 years ago, and our skill continues to increase.

A: Seasonal forecasts usually rely on much coarser numerical models than shorter-range forecasts we use every day. This is due to the large number of computations required to forecast months into the future. Also, shorter range models, with all their details and nuance, simply "blow up" if run for periods beyond their design parameters. Their output becomes useless after a few weeks into the future. Also, the things that go into a long range forecast are rather different.

The inputs to the Climate Prediction Center's long range forecasts include climate records, sea surface temperatures, the phase of well-understood oscillations like El Nino/La Nina, and many other parameters. Both statistical models and physical models are used and each has their strengths and weaknesses. Statistical models are not very good at predicting highly unusual events, and physical models are computationally intense and take lots of resources to produce.

The reason Central California can so often go "either way" is probably owing to the way both El Nino's and La Nina's impact the West Coast's weather. La Nina increases winter rainfall in the Pacific Northwest and decreases it in the Pacific Southwest. El Nino has the opposite effect. Central California is caught between the impact area of these influences. So, we have little predictive value from recognizing either a La Nina or El Nino. In fact, Central California has gotten both flood and drought from both La Nina and El Nino winters.

Seasonal forecasts are designed to provide information as to whether a season will end up being above or below average for either rainfall or temperatures. It cannot give a picture of any given day during the season. These forecasts are best used as gross indicators of the overall season and don't have much value for deciding, say, whether or not we'll have clear skies for viewing the Easter sunrise.

A: Atlantic hurricanes begin life in the lower latitudes where the prevailing winds are from the east. Over the continental US, prevailing winds are from the west. As hurricanes move farther northward, they gradually encounter the westerly winds and begin to curve in response. You can see this phenomenon pretty graphically by looking at maps of past hurricane tracks. The lines start out tracing westward, then take a turn east as the encounter higher latitudes.

Why the winds are from the east along the low latitudes is a bit of a complicated mess to explain. I'll just leave it at this...it has to do with how friction and the spin of the Earth causes the winds to turn. Close to the equator, the effect of the earth's spin is very slight. Farther northward it's much stronger.

Thanks for the question Dave.

A: Isolated showers moved across the area very early this morning. It wasn't a vivid dream.

We saw the showers on the radar. The coverage was very patchy and missed the rain gage network almost entirely. Sacramento International Airport received only a trace of measurable rain. You must have been closer to a more active area.

A: The main thing you want to avoid is biasing the temperature reading by putting the thermometer close to anything that gets much hotter or much colder than the surrounding area. Examples of things that tend to mess up temperature readings include large paved areas, rooftops, sloped terrain, marshy areas, beautiful tree groves, and poorly ventilated enclosures (like my back patio).

The ground itself will get much hotter than the surrounding air, so you want to make sure that you're neither too close nor too far from it. The standard height is four to six feet above the ground surface.

It's best if the thermometer is not located in direct sunlight, but it shouldn't be placed in an expanse of shade either. Official temperature sensors have thermal shields that keep the sun's rays from overheating the sensor material. Older style official thermometers are actually placed inside special shelters that allow for air movement but block direct sunlight. They are painted white to prevent additional heat absorption. They look like little sheds and sort of remind me of pigeon coops--which I can't actually recall ever seeing now that I think about it.

Of course, if you just want to know how hot it is in your back yard, you don't really have to worry too much about strict compliance with all these guidelines. Just place the thermometer about six feet off the ground and away from buildings--or at least on a part of the building that doesn't get direct sunlight.

Thanks for the question Don.

A: David, there are several reasons, but you really hit on the main one. The biggest reason Sacrament isn't even hotter has to do with our proximity to the ocean.

Right now the water temperature just off the Northern California Coast is around 52 degrees. That very cool water chills the air near the Earth's surface. Consequently, the air temperatures just off shore are not much warmer than the water itself--usually in the upper 50's. When winds carry this cool air inland it moderates the amount of heat we experience in the Central Valley.

Cool ocean air moving inland through the Sacramento River Delta is usually referred to as the Delta Breeze. Our hottest summer days have little or no cooling from the Delta Breeze. Our less hot summer days are usually due, at least in part, to a robust Delta Breeze. It's sort of like a big window air conditioner that cools part of the Central Valley. Locations farther away from the Delta Breeze effect experience hotter temperatures.

Another reason Sacramento isn't as hot as Phoenix has to do with our position on the planet. You probably remember from grade school that the sun's rays are most concentrated near the equator and least intense at the poles. Sacramento is about 350 miles farther from the equator than Phoenix so sunlight here is slightly less intense. I'm not sure how to quantify that effect, but it's probably not very significant. I'd guess that it makes for couple degrees difference.

There are other significant differences between Sacramento and Phoenix. One I think is frequently important is that we tend to get brushed by weak weather systems moving over the Pacific Northwest during the late spring and early summer. Cooler air masses associated with these systems can sometimes affect our high temperatures by shaving as much as 10 or 15 degrees off what we'd otherwise receive.

But, by far, the main reason has to do with the coastal influence. Sacramento is lucky to have a window air conditioner sitting just to the west--if only we could turn it on whenever we wanted.

Thanks for the question David.

A: Easy! The answer is cold ocean water.

Okay, I'll try to do a little better. Let's see, where to start. For the benefit of the many who don't know what the heck we're talking about, I'll start by saying that "convective activity" is the process for making the kind of clouds which produce thunderstorms. So I'll make my answer a bit more simplistic and just talk about why the Central Valley doesn't get as many thunderstorms as oh...say, Oklahoma.

Mother Nature serves up Thunderstorms every day on this planet. Thousands of them. There's a bunch going on right now while you are reading this. Thunderstorms are easy to make as long as you have the right ingredients. The recipe calls for three things: abundant heat, abundant moisture, and some kind of trigger (like a weather front or low pressure area). The tricky part is all of these have to come together at the same time and in the right proportions.

Actually, you need quite a bit of heat and moisture in the air. It really needs to just be pouring into lower levels of the atmosphere. The best way to get moisture into air is to move it over warm ocean water. So now you can see what we're up against here on the West Coast.

If you've ever tried to go swimming at a beach anywhere within driving distance, you for sure know that our ocean water is not warm. In fact it's pretty darn cold--usually in the 50s. Our chilly water comes from the currents off the West Coast of North America which bring cold water to the surface. Without warm, moist air in the lower level of the atmosphere, we just don't have all the ingredients handy to create lots of thunderstorms. Most of our thunderstorms happen when we get warm, moist air imported. This can happen when we get a jet stream ushering in tropical air or, more typically, when summer winds import moisture laden air in a long journey from the Gulf of Mexico. These things happen, but it's rather infrequent because we're so far away from the warm water's source.

On the other hand, Oklahoma is very near warm ocean water--thanks to its proximity to the Gulf of Mexico. Any time southerly breezes occur in Oklahoma, they usually bring in the warm, moist air from the Gulf. They get lots of thunderstorms because the ingredients are just there more often.

Thanks for the question.

A: Ah yes...what about May 2010? Well, it turns out that last month was a pretty cool one. The average temperature for the month was about five degrees below the long term normal. That is kind of a dull statistic though, so my staff at the weather office dug a little deeper.

Since daily temperature observations began in 1877, May 2010 was the 24th coldest May on record. That's a record only a weather geek could really appreciate, so here is something more interesting: it never reached 90 degrees last month. The last time that happened was 1971. Of course, that's only good for the Sacramento City climate station though, which takes readings downtown. Out at Sacramento Executive Airport, on the other hand, the temperature did in fact reach 90 on the 30th of May.

All this cool and relatively wet spring weather has been beneficial. The start of our fire season has been delayed for weeks, and our reservoirs have more water than they've had in years. In fact, if you get the chance to drive by Shasta Lake anytime soon, you'll have the treat of seeing it nearly full. That hasn't happened since 2006!


Thanks for the question Kevin.

A: We definitely have been in a cool pattern for several weeks. The meteorological reason has been the frequent presence of a large upper level low along the West Coast of North America. It has been sending cool, wet weather over us. What causes such a pattern to form and persist is probably beyond my ability to understand, let alone explain. But there is one thing I could pass on--our weather is linked to all the weather on the planet.

When our weather is colder than usual chances are someone else has unusually warm weather. The jet stream driving our weather systems is a little like a seesaw. If it's up on one side, it's got to be down on the other. So, while we have been having cool, wet weather in recent weeks, the Northeast and North Central US has been having unusually warm weather. Turns out April 2010 was the second warmest on record for parts of the Great Lakes Region. That's kind of how these things work. I was in New York City last month and got to experience some of that record heat, and let me tell you it wasn't what you'd call a dry heat.

So what about the summer forecast? Well, the extended forecast for the first part of our summer is for a return to more normal seasonable conditions. However, embedded in all that "normal" will be some very hot days and some cooler than usual ones. That forecast is indeed backing off from predictions earlier in the season of an increased likelihood of a hotter than average summer.

Is the volcano doing this? Probably not. The amount of ash going into the upper atmosphere isn't enough to affect the global temperature significantly. Besides, if there were an effect it should be distributed fairly uniformly across the planet, and there were plenty of places which have been hotter during past weeks.

Thanks for the question.

A: Yeah, there are bunches of them. Currently, my favorite way to tell if was raining on a particular date is to check out this site:

http://www.climate.gov/#climateWatch

Scroll down on the page and look for the "Past Weather" feature. It's in green letters and has a faint image of the continents. You can enter a city and a date and get the weather for that specific date. Pretty cool.

A: Yeah, it could be. So far we haven't hit any 90's this month and it doesn't look like we will any time soon. The last time we had a month of May with no 90's was . . . wait for it . . . 1971! With one third of the month remaining it's still too early to tell if this is going to be one for the record books, but it's well within the realm of the possible.

The forecast for the weekend and well into next week is for continued unseasonably cool weather. That's handy for heat haters and will delay the onset of fire season over much of Northern California.

Thanks for the question.

A: The wind speed and direction depends on many things, but the two biggest considerations around here are the temperature distribution across the Central Valley, and the location and strength of weather systems.

During fairly benign conditions, the dominate influence comes from the temperature distribution. When hot weather develops over the Central Valley, air near the surface rises due to the simple fact that hot air is less dense than cooler air. Meteorologist call these thermal lows or thermal troughs.

It's a fact of life that nature, and my cat, abhors a vacuum. So when air rises from the surface due to heating, the adjacent air rushes in to replace it. This happens on a fairly large scale in the Sacramento Valley during the summer. Air that rushes in to replace it comes through gaps in the coastal mountains. Near our area the wind comes up through the Sacramento Delta--hence the name "Delta Breeze". The temperature differences that drive the Delta Breeze peak in the late afternoon, which is why the Delta Breeze always seems to arrive around here in the late afternoon or early evening.

When strong weather systems are moving through the area, their pressure patterns usually overwhelm any temperature induced winds. Winds blow toward an area of low pressure and away from an area of high pressure. As a front crosses the area, we tend to get southeast winds ahead of the front and west or southwest winds behind it. Strong high pressure to our north will produce northwesterly winds in our area. By the way, the direction the air is coming from is the wind direction. So a "west" wind is air coming at you from the west.

The best way to tell what is going on is to check the current weather observations and forecast. You can easily do that at the National Weather Service web site: www.weather.gov/sacramento.

Thanks for the question Danny!

A: Cool question. Baseball-sized hail is not something you get very often, but it does happen. I have held baseball-size hailstones in my hands (and have pictures to prove it!).

The atmosphere is capable of doing some awesomely powerful things. The amount of energy released by a large thunderstorm is comparable to an atomic bomb. Fortunately, this energy is released over a much longer time span.

Very large hail is relatively rare in Northern California, but certainly not unheard of. In a typical season, we see a thunderstorm or two producing hail up to an inch or more in diameter. One of the interesting things about our area is we more often get smaller diameter hail falling to a very great depth. This has to do with the origin of the air masses that spawn our thunderstorms, and their slow movement across the terrain. I've seen photos of hail as deep as a foot--from a single thunderstorm! This kind of weather is really dangerous. Imagine driving along I-5 and coming upon hail that's 6 inches deep! It definitely pays to keep an eye on the sky.

Thanks for another great question.